This invention relates to apparatus for moving shutter blades, filter elements or other components of photograhic copying machines or the like between spaced-apart first and second end positions and for holding such components in selected end positions. More particularly, the invention relates to improvements in apparatus wherein a mechanical component (such as a shutter blade or a filter element) is movable between and can be held in either of two end positions by magnetic force.
It is already known to resort to magnetic fields in order to move filter elements and/or shutter blades of copying machines between first and second end positions. If the mobile component is a shutter blade, it allows light to pass through the shutter in one end position and prevents the passage of light in the other end position thereof. A filter element extends across the path of copying light in one end position and is remote from such path in the other end position. In many instances, the component is movable by means of a rotary electromagnet which continuously accelerates the component during movement from the one to the other end position (because the width of the magnetic gap decreases); the component is brought to an abrupt halt by a fixed stop against which the component abuts in the other end position. Such apparatus are not entirely satisfactory because the impingement of mobile component against the stop generates noise and the component is likely to rebound on the stop which can result in undesirable secondary exposure to light if the mobile component is a shutter blade.
It was further proposed to move filter elements or shutter blades by means of linear motors. The current in the moving coils of the linear motor must be regulated with a high degree of accuracy, and this can be achieved only by resorting to extremely complex, bulky, expensive and highly sensitive control circuits. Such circuits must insure that the filter element or shutter blade is accelerated during the first half and is gradually decelerated during the second half of movement from the one toward the other end position. Therefore, such linear motors failed to gain widespread acceptance in the art of copying machines in spite of the fact that they can insure rapid and controlled movements of filter elements or other mobile components between two spaced-apart end positions.